When the “Check token uniqueness” option is enabled, every time a token is created the gateway checks the uniqueness of the payment method. If the payment method has already been registered (same number and expiration date), then the vads_identifier_previously_registered field is transmitted with the true value in the end of payment notification.
The vads_identifier_previously_registered field is never returned in the end of payment notification if no duplicate payment methods are detected. Therefore, the false value is never sent to the merchant website.
Output field, returned in the response (IPN only).
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